1. Field of the Invention
The present invention relates to the field of leak detection in pipelines. More particularly, the present invention relates to a method and system for integrated acoustic leak detection. Even more particularly, the present invention relates to an improved method and apparatus for quick, sensitive, and accurate detection and location of the source of a leak in a pipeline utilizing the combination of easily installed non-intrusive sensors and the highly sensitive and reliable acoustic sensors leak detection techniques.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
In pressurized systems, such as pipelines, the pressure boundary is maintained by the pipe wall. The pipe wall has a yield stress greater than the stress exerted from the system pressure. At the instant of a breakdown of the pressure boundary of the pipe wall, the release of the elastic force couples with the system fluid to create a transient pressure wave. Since pressure is relieved (due to the break in the pipe wall) from the containment system, the transient pressure wave takes the form of an expansion wave. This expansion waves travel outward in all directions from the source at the speed of sound for that fluid. When the fluid is contained in a pipeline, the expansion waves are guided through the fluid by the walls of the pipe in either direction from the source of the break.
Real time acoustic pipeline leak detection requires placing permanent monitors on a pipeline for detecting expansion pressure waves associated with a sudden break down of the pressure boundary. In the past, it was the pattern and amplitude of the signal that were of concern. The source of the pressure waves was located between monitors by recording the times when the expansion pressure wave arrived at least two different monitors. Using these times (t2 and t1), knowing the fluid sound velocity (V) and the length of pipe between monitors (D) the leak event could be located. As shown in the following equation where X is the leak event location (distance from sensor number 1).
If this event was not located between the monitors, it was ignored as a false event coming from outside of the protected zone. Previously, background noises were filtered out by various techniques such as moving average, repetitive filter, dynamic threshold and band pass filters. Although, these have successfully filtered out certain types of background noises, these techniques have little affect on other types of transient noises, such as noises from pumps, compressors, and valve operations. These transient noises oftentimes produced signals with patterns and amplitudes similar to the patterns and amplitudes of such signals produced by leaks. This has led to a high false alarm rate and reduced sensitivity.
Various patents have issued in the past relating to leak detection in pipelines. One of the present inventors is an inventor on several patents in the field. For example, U.S. Pat. No. 6,389,881 issued on May 21, 2002 to Yang, et al. describes a method and apparatus for pattern match filtering for real time acoustic pipeline leak detection and location. The patent describes how pattern match filtering is used to reduce false alarm rate, increase sensitivity and improve leak location accuracy, while quickly detecting leaks by the acoustic signal generated from a leak event in pipelines containing gas or liquid under pressure. The pattern match filter technique detects a pressure wave generated by a leak, but discriminates against background noise and pressure disturbance generated by other non-leak sources that might otherwise be detected as a leak. The pattern match filter derives a sharp peaked output from the signal of the expansion wave which allows for a distinctive point of reference for a time stamp. This provides for improved accuracy in leak location calculations. The pattern match filter is incorporated into site processors located at multiple points along a pipeline, and at a central node processor which receives data from all site processors. The pattern match filter includes using previously recorded leak profiles. At site processors located at multiple points along a pipeline, a series of previously recorded signature leak profiles are continuously compared in real time against pipeline pressure signals. Data from each site processor are used collectively at a node processor and compared against multiple leak profiles to provide further false alarm rejection. The leak event data generated at each site processor is used by the node processor to declare a leak. By the application of this pattern match filter technique, the signal to noise ratio (S/N ratio) required to identify a leak event is reduced and the sensitivity of leak detection is increased. U.S. Pat. No. 6,668,619 issued to Yang et al. on Dec. 30, 2003 describes a related method of pattern match filtering.
U.S. Pat. No. 6,301,973 issued on Oct. 16, 2001 to Smith and describes anon-intrusive pressure sensor and method. In the patent, non-intrusive pressure sensors for measuring unsteady pressures within a pipe include an optical fiber wrapped in coils around the circumference of the pipe. The length or change in length of the coils is indicative of the unsteady pressure in the pipe. Bragg gratings impressed in the fiber may be used having reflection wavelengths that relate to the unsteady pressure in the pipe. One or more sensors may be axially distributed along the fiber using wavelength division multiplexing and/or time division multiplexing.
A strain gauge is an instrument used to measure strain on structures subjected to the action of external forces. Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length (L): Strain (ε)=ΔL/L. Typical values for strain are less than 0.005 inch/inch and are often expressed in micro-strain units: strain×106.
In particular, pipelines and vessels are subjected to various external forces that produce strain in different geometrical directions, e.g. longitudinal and hoop, to the pipeline or vessel wall. Such external forces are varied in nature and their presence and magnitude depend on the installation, environmental and operating conditions of such pipelines or vessels. Examples of these forces are soil in underground pipelines or liquid loads in underwater pipelines, fluid pressure, bending forces of any structure linked mechanically to the pipe or vessel, such as piping, mechanical supports and valves, as well as mechanical vibration and other transient mechanical forces.
When the pipeline or vessel wall suffers the prolonged action of such forces, various defects can occur in their material structure such as micro cracks, fissures and defects of the kind Eventually, these defects can lead to major cracks and full rupture.
In metals, more in particular in metallic pipelines and metallic vessels, such defects may be present in conjunction with internal and/or external corrosion and fatigue caused by operational conditions, meaning fluctuations in the operating pressure and temperature, ambient temperature changes, changes in mechanical load applied to the structure, such as the soil movements in underground pipelines and under water currents in sub-sea pipelines. The combined effect of corrosion and fatigue, among other deteriorating effects to pipelines and vessels not specifically mentioned but not excluded here, weaken the pipeline or vessel wall.
In particular, the operating pressure is manifested on the pipeline or vessel wall as a measurable strain on the pipeline or vessel wall, as axial and hoop strain. Axial and hoop strains exist simultaneously at each cross section of the pipe. The pipe wall stretches in the circumferential and axial directions the same amount at the same time.
Fluctuations in the operating pressure in pipelines and vessels occur due to various causes, including increase or decrease of demand in pipelines supplying fluid to customers, process disruptions from malfunctioning compressors or pumps or caused by operators, changes of process conditions, sudden expansion of compressible fluids, start and stop of pumps and compressors, opening of relief valves, action of pressure and flow control valves, and many other transient conditions produced by mechanical equipment mechanically linked to the pipeline or vessel, as well as transient events such as pipeline leaks. These pressure fluctuations produce pressure waves of various patterns that travel across the fluid as well as the pipeline and vessel wall. Such pressure waves manifest as a measurable strain reading on the pipeline or vessel wall.
In pipelines leak detection systems, the previous patented technique [Yang & Recane] for filtering undesirable signals, generally coming from external sources placed beyond pipeline limits or beyond the protected segments of the pipelines, such as pumps and compressors, was based on the detection of the transit direction of fluid waves by means of a dual intrusive sensor arrangement whereas each sensor probe is physically exposed to the pipeline fluid. The intrusive types of sensors, such as pressure and temperature sensors, are conveniently placed at the end of the pipeline. In this kind of solutions, the dual sensor arrangement is connected to a field processor capable of determining the difference in the arrival time of the fluid waves. With proper wavelength and sensor span, when accurately determined, the difference in the arrival time of such waves and its sign allows the processor to determine the transit direction of such wave.
The main limitation in the use of intrusive sensors on pre-existent pipeline facilities resides in the fact that the sensor needs to be in direct contact with the pipeline fluid and consequently expensive and risky hot-tapping techniques may be required when tapping points are not available. This limitation imposes a high total installation cost and risk in pipeline projects.
Various experiments have been conducted in order to determine the strain effects of pressure waves by measuring strain as a function of the maximum strain rate in a pipeline. It has been verified that strain waves also add and subtract to the maximum strain. The magnitude of the maximum strain varies as a function of strain wave effects. The variance in the measured maximum strain rate is attributed to the strain effect of pressure waves. This strain resulting from pressure waves traveling along a pipeline originates a resultant strain wave, which occur throughout the pipe length at a value proportional to the maximum strain.
In particular, one of the sources of pressure waves is a leak in a pipeline or vessel. Pipeline or vessel leaks produce negative pressure waves which travel in all directions across the fluid contained in the pipeline or vessel. These pressure waves attenuate as they move away from the leak-origin point. The attenuation may be significant at the high frequency wave components. The resulting strain wave measured on the pipeline or vessel wall correlates to the leak pressure wave. The correlation between pressure waves and strain waves on a pipeline can be observed in various experimental results.
However, in presence of vibration caused by fluid or external sources the leak-strain wave intensity in relation to the vibration signal intensity at a given point on the pipeline or vessel require specific leak pattern recognition techniques to be detected.
In process plants, piping and vessels are exposed to various sources of vibration as well as pipelines are exposed to fluid and mechanical vibration. Thus, the leak-correlated strain wave measured at different points of the pipeline or vessel wall provides an indication of the magnitude of the leak pressure wave. However, in order to estimate the measured strain into a meaningful fluid pressure measurement, a reference pressure point must be provided as well as temperature, fluid and pipeline or vessel wall material properties must be known. This is because the strain response of the pipeline or vessel wall material to the fluid pressure energy varies with the ambient temperature, fluid and material properties. An estimation of pressure wave as a function of the resulting strain on a structure requires a physical model that represents the acting physical variables of the phenomenon. If necessary, an on-site fine tuning can be performed with intrusive acoustic pressure sensors as well as non-intrusive sensors to correlate the patterns.
For further accuracy in the fluid pressure calculation as a function of measured strain, the physical model includes a thermal model that represents the thermal expansion of the materials, capable of inferring fluid temperature by taking into account the temperature of the medium surrounding to the pipeline or vessels, such as soil temperature, air ambient temperature or the surrounding liquid temperature in the case of submersed pipelines.
Most of the heat loss in a pipeline or vessel occurs in the heat conduction between the pipeline or vessel wall to the surrounding media. Assuming heat conduction is the primary heat transfer mechanism, the heat transfer between the pipeline or vessel wall to the surrounding media is estimated by the Fourier law as function of the temperature gradient between the temperature of the pipe wall and the surrounding media. For a subsea pipeline, heat transfer due to convective heat transfer might also have contribution to the temperature of the pipe wall. Another thermal phenomenon is the thermal expansion of the pipeline or vessel wall in presence of varying temperature conditions. The resultant temperature of the pipeline or vessel wall defines the effect on strain measured on the structure.
In addition to the presence of vibration, the measurement of strain waves on pipeline or vessel walls by means of electrical resistance strain gauges suffers problems of poor signal-to-noise ratio due to the electrical nature of the measurement, which can be affected by surrounding electrical currents or induced currents through electromagnetic sources, or resulting from capacitive or resistive coupling with other sources, such underground electrical power lines, current discharges to the ground grid (ground potential raise-GPR), and among other electrical-related phenomena including ambient conditions (lightening).
Traditional foil strain gauges do not have adequate signal-to-noise ratios at such small strains. Specially designed fiber-optic strain gauges, as well as special resistance-based strain gauges, have been shown to be potentially useful for measuring such small strains. There are various methods to measure strain on materials based on laser light propagating across a fiber optic attached to the structure on which strain wants to be measured. In presence of strain forces the light emerging from one end of the fiber forms a speckle pattern that changes as strain is applied to the structure. The speckle pattern is intercepted by an array of photocells, so that any change in the speckle pattern manifests itself in changes in the intensities of light measured by the individual photocells. The outputs of the photocells are collected by a customized expert based data-acquisition system that includes a uniquely configured signal-conditioning subsystem. The photocell outputs are then fed to a neural network or pattern recognition system similar to the one described in the previous patent [Yang et al.] that recognizes the correlation between changes in the outputs and changes in strain as a result of different transient events, such as leaks. Inasmuch as the changes in the intensities of light incident on the photocells are repeatable for a given amount of change in strain, the neural network can be quickly trained by use of speckle patterns associated with known patterns and profiles of strain. For measurement of temporally varying strain (for example, when vibrations are present), the update rate and, hence, the dynamic analysis rate depends on the data-acquisition rate.
It is an object of the present invention to provide an acoustic leak detection system that utilizes both intrusive and non-intrusive sensors.
It is another object of the present invention to provide an acoustic leak detection system that utilizes the measurement of strain on a pipeline or vessel to detect leaks.
It is another object of the present invention to provide an acoustic leak detection system and method that is relatively inexpensive to install and maintain.
It is another object of the present invention to provide a leak detection system and method that filters background noise.
It is another object of the present invention to utilize specially configured and trained neural networks or pattern recognition methods similar to the one described in the previous patent [Yang et al.] to identify the unique strain pattern as measured by various non-intrusive devices.
It is yet another object of the present invention to utilize the GPS time tagging approach based on leak event detecting times registered by either intrusive or non-intrusive sensors at two or more different locations along the pipeline to calculate the precise leak location based on time of flight relationship.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification.